Promoting Proton Donation through Hydrogen Bond Breaking on Carbon Nitride for Enhanced H2O2 Photosynthesis

Photocatalytic H2O2 production has attracted much attention as an alternative way to the industrial anthraquinone oxidation process but is limited by the weak interaction between the catalysts and reactants as well as inefficient proton transfer. Herein, we report on a hydrogen-bond-broken strategy in carbon nitride for the enhancement of H2O2 photosynthesis without any sacrificial agent. The H2O2 photosynthesis is promoted by the hydrogen bond formation between the exposed N atoms on hydrogen-bond-broken carbon nitride and H2O molecules, which enhances proton-coupled electron transfer and therefore the photocatalytic activity. The exposed N atoms serve as proton buffering sites for the proton transfer from H2O molecules to carbon nitride. The H2O2 photosynthesis is also enhanced through the enhanced adsorption and reduction of O2 gas toward H2O2 on hydrogen-bond-broken carbon nitride because of the formation of nitrogen vacancies (NVs) and cyano groups after the intralayer hydrogen bond breaking on carbon nitride. A high light-to-chemical conversion efficiency (LCCE) value of 3.85% is achieved. O2 and H2O molecules are found to undergo a one-step two-electron reduction pathway by photogenerated hot electrons and a four-electron oxidation process to produce O2 gas, respectively. Density functional theory (DFT) calculations validate the O2 adsorption and reaction pathways. This study elucidates the significance of the hydrogen bond formation between the catalyst and reactants, which greatly increases the proton tunneling dynamics.

−5 The industrial production of H 2 O 2 is predominantly achieved by the anthraquinone cycle process. 6he process involves four steps, which are hydrogenation of anthraquinone in an organic solvent, oxidation of the hydrogenized anthraquinone in an O 2 -rich environment, extraction of the generated H 2 O 2 and recycling of anthraquinone, and purification and concentration of H 2 O 2 . 7H 2 O 2 is produced in the oxidation process of the hydrogenized anthraquinone.This industrial process is capable of largescale production.Nevertheless, this multistep process consumes a large amount of energy.The hydrogen gas utilized in the hydrogenation of anthraquinone is difficult to handle and store.The generation of byproducts from anthraquinone and hydrogenized anthraquinone necessitates the use of a large amount of organic solvents, resulting in waste generation.
Various alternative approaches have been developed to overcome the challenges in the current industrial production process.Among these approaches, light-driven H 2 O 2 synthesis has been emerging as one of the most appealing methods for H 2 O 2 production. 8,9Photocatalytic H 2 O 2 production typically includes several fundamental steps: light harvesting; generation, transfer, and separation of photogenerated electron− hole pairs; O 2 and/or H 2 O adsorption; surface redox reactions; and H 2 O 2 desorption. 10Notably, this process relies exclusively on four key components: catalysts, O 2 , H 2 O, and solar energy.In addition, sacrificial agents are required for most reported catalysts. 11Two-electron oxygen reduction reaction (ORR) and two-electron water oxidation reaction (WOR) represent two promising pathways for achieving photocatalytic H 2 O 2 production. 12−14 Among these pathways, the two-electron ORR has mainly been investigated for H 2 O 2 production due to its superior thermodynamic favorability compared to the twoelectron WOR.The ORR process can be classified into direct one-step two-electron reduction (eq 1) and indirect two-step single-electron reduction (eqs 2 and 3).The thermodynamics for H 2 O 2 shows that the reaction potential of the one-step twoelectron ORR is more positive than that of the two-step oneelectron pathway.The direct ORR is thus thermodynamically favorable.In contrast, the direct ORR is less kinetically favorable because of two-electron transfer compared with the indirect pathway, which is only one-electron transfer for each step.For the indirect ORR, the reduction of O 2 to superoxide radical (•O 2 − ) is the rate-determining step since its reaction potential is more negative.The generated intermediate •O 2 − might react with organic compounds in the reaction system or be reoxidized by hot holes to produce singlet 1  (2) However, the photocatalytic H 2 O 2 production is limited by the weak interaction between the catalyst and O 2 gas as well as inefficient proton-coupled electron transfer.In most protonation processes, H 2 O molecules are utilized as the proton source.Proton donation is restricted by the sluggish reaction kinetics in the oxidation of H 2 O molecules. 15In addition, the protons are easier to combine into H 2 gas, which further limits the H 2 O 2 production efficiency.Photocatalysts employed for H 2 O 2 production can be broadly classified into three types, namely, metal oxides, 16 organic materials, 17−19 and carbon nitrides. 20,21Among these types of photocatalysts, g-C 3 N 4 has emerged as a widely employed photocatalyst for photocatalytic H 2 O 2 production owing to its advantageous features, including cost-effectiveness, facile synthesis, appropriate band gap structure, thermal and chemical stability, and environmentally friendly nature. 22−27 The photocatalytic efficiency of g-C 3 N 4 in H 2 O 2 production has been demonstrated, 28 wherein the formation of 1,4-endoperoxide species on the g-C 3 N 4 surface suppresses the undesired one-electron ORR to •OOH but preferentially accelerates the selective two-electron ORR to H 2 O 2 . 25,29The selectivity of two-electron ORR toward H 2 O 2 production over g-C 3 N 4 is therefore high.As a typical layered material, g-C 3 N 4 exhibits strong intralayer chemical bonding and weak interlayer van der Waals interactions (Figure 1a,b), 30 which has been characterized to be shown as two diffraction peaks at 13.1 and 27.2°in the X-ray diffraction (XRD) pattern (Figure 1c), respectively. 31Nevertheless, despite the dominance of covalent bonding within the intralayer framework, abundant hydrogen bonds between the NH/NH 2 groups at the edge of the melon strands exist owing to the incomplete polymerization of the nitrogen-containing precursors. 32,33hese hydrogen bonds serve as binders to interconnect the strands of the melon units and maintain the long-range atomicorder pattern within the intralayer framework.However, the existence of intralayer hydrogen bonds on carbon nitride also leads to the deficiency of exposed N atoms, which hinders the hydrogen bond formation between exposed N atoms on carbon nitride and H 2 O molecules.N atoms on carbon nitride are supposed to be proton buffering sites.The deficiency of exposed N atoms therefore hinders the proton transfer from adsorbed H 2 O molecules to carbon nitride.Moreover, the presence of hydrogen bonds makes charge carrier transport difficult within the intralayer framework of g-C 3 N 4 because of a large electrostatic potential barrier of 7.9 eV across the intralayer hydrogen-bond-located regions, thereby impeding intralayer charge carrier transport and therefore photocatalytic activities. 34Therefore, breaking the intralayer hydrogen bonds in g-C 3 N 4 plays an important role in the enhancement of the photocatalytic activity.
The importance of the hydrogen bond formation between catalysts and reactants in catalysis has been demonstrated in several intriguing studies.For instance, hydrogen bond interactions in nitrogen photofixation have been found to enhance N 2 activation.Notably, S atom-modified porous Cu catalysts exhibit high catalytic activities, where the formed S− H bonds facilitate N 2 activation through hydrogen bonding. 35oreover, the proton activity within the interfacial layer is of vital importance to proton-coupled electron transfer kinetics.Modification of the local proton activity with various protic ionic liquids in the interfacial layer of Au and Pt has been shown to substantially enhance the oxygen reduction activity, exhibiting a volcano relationship with the pK a of the ionic liquid.This enhancement is ascribed to the favorable protoncoupled electron transfer (proton tunneling) kinetics facilitated by enhanced hydrogen bonding between the ORR products and the ionic liquid. 36Hydrogen bonds, both on the catalyst surface and in the bulk solution, offer valuable opportunities for tailoring the reaction pathway for selective photocatalysis.In the photocatalytic dehydrocoupling of ethanol over the Au/ CdS catalyst, the presence of hydrogen bonds inhibits the oxidation and reverse reaction of α-hydroxyethyl radicals.Manipulating hydrogen bonds through water addition promotes the hydrogen bond formation between adsorbed αhydroxyethyl radicals and H 2 O molecules, strengthens the hydrogen bonding between α-hydroxyethyl radicals and ethanol, and thus facilitates the desorption of α-hydroxyethyl radicals from the catalyst surface. 37Considering the crucial role of hydrogen bonds in catalysis, investigating their effect on H 2 O 2 photosynthesis over carbon nitride becomes highly desirable.The enhancement of hydrogen bonding between the catalyst surface and reactants holds promise for enhancing photocatalytic H 2 O 2 production.
In this work, the interaction between the catalyst surface and reactants during H 2 O 2 photosynthesis is enhanced through the intralayer hydrogen bond breaking in carbon nitride.The breaking of the intralayer hydrogen bonds in carbon nitride is realized through post-thermal treatment in an inert atmosphere.The hydrogen-bond-broken carbon nitride photocatalyst exhibits an extensively enhanced H 2 O 2 photosynthesis efficiency.The reaction mechanism studies show that O 2 molecules are reduced to H 2 O 2 by photoinduced electrons through a one-step two-electron reduction pathway, while H 2 O molecules are oxidized to O 2 molecules by photoinduced holes through a four-electron oxidation pathway.Importantly, the decomposition of H 2 O 2 is mitigated in the hydrogen-bondbroken carbon nitride.The study shows that the hydrogen bond breaking in carbon nitride leads to the formation of NVs and cyano (−C�N) groups, which are both active sites for the adsorption and reduction of O 2 gas toward H 2 O 2 photosynthesis.The hydrogen-bond-broken carbon nitride is also found to have more exposed N atoms, which can form hydrogen bonds with H 2 O molecules.The formation of hydrogen bonds between exposed N atoms in the hydrogen-bond-broken carbon nitride and H 2 O molecules greatly contribute to the enhancement of proton-coupled electron transfer from the adsorbed H 2 O molecules to carbon nitride.In other words, the exposed N atoms in hydrogen-bond-broken carbon nitride provide proton buffering sites, which promotes the proton donation.

RESULTS AND DISCUSSION
Photocatalyst Synthesis and Characterization.The intralayer hydrogen bond breaking in carbon nitride was realized by subjecting the material to post-thermal treatment under an inert atmosphere, as illustrated in Figure 2a.The bulk g-C 3 N 4 (referred as C 3 N 4 -hb) was synthesized through the calcination of melamine in an air environment.The asobtained C 3 N 4 -hb sample was subsequently ground into powder and further thermally exfoliated to form C 3 N 4 -hb nanosheets.These C 3 N 4 -hb nanosheets were finally subjected to thermal treatment under inert Ar gas at various temperatures and durations.The treatment temperatures ranged from 520 to 650 °C, and the durations ranged from 2 to 10 h.The morphologies of the treated carbon nitride samples exhibit clear changes associated with the disruption of intralayer hydrogen bonds, as revealed by the transmission electron microscopy (TEM) images (Figure 2b and Figure S1).The pristine C 3 N 4 -hb possesses a smooth surface, whereas the thermal treatment induces the formation of abundant pores with relatively uniform sizes.Notably, at 520 °C, the thermal treatment results in the formation of slit holes with uniform widths.These slit holes share one orientation and are nearly parallel to each other.The formation of these slit holes along the strands is attributed to the volume shrinkage of the carbon nitride owing to the partial breaking of the hydrogen bonds between the melon strands.The single orientation of the strands determines the single orientation of the formed slit holes.With increases in the treatment temperature, a higher portion of the intralayer hydrogen bonds are broken.In addition, the interconnected pore walls gradually become thinner as the treatment temperature is increased.However, at temperatures exceeding 650 °C, some pore walls collapse owing to the thermal decomposition of the melon strands.The pore size exhibits slight changes, while the specific surface area gradually increases from 10.4 to 128.6 m 2 g −1 (Figures S2 and  S3).This structural modification of C 3 N 4 results in a significant increase in the sample volume for a given mass (Figure 2c).Moreover, the color of C 3 N 4 changes from light yellow to dark orange, accompanying the structural modification (Figure 2c), which is advantageous for efficient solar light harvesting.
The evolution of the XRD patterns of the carbon nitride samples with the treatment temperature and duration is displayed in Figure 2d.As mentioned above, the peaks observed at 13.1 and 27.2°can be indexed to the in-planeordered tri-s-triazine motifs and the interlayer stacking of the aromatic systems, respectively.The peak centered at 13.1°g  radually diminishes as the treatment temperature is increased, indicating the disruption of the intralayer periodicity of the tris-triazine units and the progressive breaking of the intralayer hydrogen bonds.Simultaneously, the peak centered at 27.2°b ecomes broader and weaker owing to the fluctuation of the intralayer structure and disruption of the periodic stacking of the layers, resulting from the breaking of the intralayer hydrogen bonds.At higher treatment temperatures, the longrange periodicity of C 3 N 4 is completely disrupted, giving rise to a relatively broad peak in the XRD patterns.To further investigate the chemical structures of the different C 3 N 4 samples, Fourier transform infrared spectroscopy (FTIR) was employed.All the samples exhibit several similar broad characteristic absorption bands (Figure 2e).The band observed at 808 cm −1 is attributed to the breathing mode of the triazine units, while the band ranging from 1200 to 1900 cm −1 arises from the stretching vibrations of the aromatic CN heterocycles.The broad absorption band in the range of 2400 to 3650 cm Based on the distinct characteristics revealed by these samples, three representative samples were selected for further comparison: bulk g-C 3 N 4 with intralayer hydrogen bonds but no NVs, C 3 N 4 treated at 520 °C for 8 h with intralayer hydrogen bonds and NVs, and C 3 N 4 treated at 650 °C for 8 h with NVs but complete hydrogen bond breaking.These samples are referred to as C 3 N 4 -hb, C 3 N 4 -hb-nv, and C 3 N 4 -nv, respectively.Their names were also defined according to the following characterization results.X-ray photoelectron spectroscopy (XPS) was employed to examine their surface compositions.Their XPS spectra all exhibit three distinct sharp peaks centered at ∼289, 400, and 534 eV, which can be ascribed to C 1s, N 1s, and O 1s, respectively (Figure S4).Peaks for NVs were not observed in the XPS spectra, which might be caused by the poor surface charging effect of carbon nitride or the measurement limitation. 38,39The C 1s and N 1s peaks were subsequently deconvoluted (Figure 3a,b).The N 1s peaks were deconvoluted into three peaks, which correspond to bicoordinated N atoms (N 2C ), tertiary nitrogen N−(C) 3 groups (N 3C ), and NH x groups.Similarly, the C 1s peaks were also deconvoluted into three peaks, representing C−C, (C) 3 −N, and bicoordinated C atoms (C 2N ).The percentages and ratios of the three peaks in the N 1s and C 1s XPS spectra were determined (Tables S1 and S2).respectively.These two main peaks are observed for C 3 N 4 -hb, C 3 N 4 -hb-nv, and C 3 N 4 -nv, which confirms that intralayer hydrogen bond breaking does not change the main structure of carbon nitride.In addition, three weak peaks appear at 120 ppm for C1 (−C�N groups), 169 ppm for C4 (−NH−C� N), and 172 ppm for C5 (�N−C�N) on C 3 N 4 -nv.This result indicates that intralayer hydrogen bond breaking leads to the deprotonation of the C−NH 2 groups and introduce −C� N groups, which is consistent with the results from FTIR and XPS.Electron energy loss spectroscopy (EELS) measurements were thereafter performed to characterize their chemical structures (Figure 3f).In the EELS spectra of the three samples, both the C K and N K edges show σ* and π* resonances.In the C K edge spectra, two peaks are ascribed to the 1s to σ* and 1s to π* electronic transitions of sp 2hybridized carbon.The two peaks in the N K edge spectra is believed to arise from the N atoms sp 2 -hydridized with C atoms and N atoms, respectively. 40The resonance peaks of C 3 N 4 -nv are both blueshifted, suggesting that the valence numbers of the sp 2 -hydridized N and C atoms become higher.The increased valence numbers of the sp 2 -hydridized N and C atoms are caused by lower valence electron densities, which indicates electron transfer from the pyridinic nitrogen (sp 2hydridized N atoms) to the electron-withdrawing groups, such as −C�N groups and NVs.Based on these findings, the chemical structures of these three samples can be derived (Figure 3g).In The chemical structure modification induces significant changes in the electronic structure of the carbon nitride samples, thus resulting in alterations in light absorption, charge transport, and charge separation.The light absorption spectra of the different C 3 N 4 samples are depicted in Figure 4a and Figure S6.In comparison with C 3 N 4 -hb, the thermally treated samples exhibit substantially reinforced light absorption, with a pronounced redshift of the absorption edge into the nearinfrared region as the treatment temperature and duration are increased.Moreover, in C 3 N 4 -hb, the light absorption is attributed to the intrinsic electronic transition from π to π*.The enhanced π−π* electronic transition can be ascribed to the expanded π-conjugated aromatic framework and better packing of the joint heptazine system.Notably, in C 3 N 4 -nv, a new absorption peak around 490 nm emerges, which is attributed to the n to π* electronic transition of the lone pair electrons at the defect sites (−C�N) (Figure 4a). 41Their electronic band gap was determined using the Tauc relationship derived from the absorption spectra (Figure 4b).The calculated band gaps are 2.38, 2.38, and 1.65 eV for C 3 N 4 -hb, C 3 N 4 -hb-nv, and C 3 N 4 -nv, respectively.The XPS valence band (VB) spectra in Figure 4c 4d).Combined with the band gap, the conduction band (CB) edge positions of the three photocatalysts were determined to be −0.29,−0.51, and −0.04 V against NHE.The obtained CB positions are in good agreement with the values obtained from the Mott−Schottky curves except for C 3 N 4 -nv (Figure S7).The reason why the CB from the Mott−Schottky measurement for C 3 N 4 -nv is different from that obtained from the XPS VB spectra might be the inaccuracy of the Mott−Schottky measurement, which might in turn be caused by the preparation of electrodes and the measurement setup.The XPS VB measurement is highly believed to be more reliable for determining band-edge positions.In addition, the band-edge position obtained from the XPS VB spectra is also in good agreement with the required band-edge position for H 2 O 2 production from the one-step two-electron ORR.The CB edge is more negative than the redox potential of O 2 /H 2 O 2 , while the VB edge is more positive than the redox potential of O 2 / H 2 O (Figure 4e).This observation indicates that the photogenerated charge carriers in C 3 N 4 -nv possess sufficient energy to drive both the two-electron ORR and four-electron WOR.
The chemisorption of O 2 molecules is an essential step in the ORR process.To assess the O 2 adsorption capability, O 2 temperature-programmed desorption (O 2 -TPD) was performed (Figure 5a).Before the TPD measurement, thermogravimetric analysis (TGA) was performed to determine the appropriate temperature for the TPD setup (Figure S8).For all the three samples, two desorption peaks were observed.The lower-temperature desorption peak corresponds to physically adsorbed O 2 , while the higher-temperature one belongs to chemically adsorbed O 2 .The chemisorption of O 2 on C 3 N 4 -hb is originated from the formation of 1,4-endoperoxide species through the adsorption of O 2 on carbon nitride, as reported in previous studies. 25,29In comparison with C 3 N 4 -hb, both the intensities of the physisorption and chemisorption peaks on C 3 N 4 -hb-nv and C 3 N 4 -nv are increased.Among them, C 3 N 4hb-nv exhibits the strongest physisorption, which might be associated with its superior pore size and volume.C 3 N 4 -nv possesses the largest relative intensity ratio of chemisorption to physisorption, which is consistent with the presence of rich defects, including both NVs and −C�N groups, formed in C 3 N 4 -nv.In contrast, C 3 N 4 -hb-nv only has a small number of NVs, which leads to its weaker chemisorption.
In addition, NVs were confirmed to exist in C 3 N 4 -hb-nv and C 3 N 4 -nv by XPS spectra (Figure 3).To further investigate the NVs, low-temperature electron paramagnetic resonance (EPR) analysis was conducted.A characteristic EPR signal with a g factor of 2.003 was observed (Figure 5b  abundant NVs shows greatly enhanced charge transfer and separation efficiency behaviors. H 2 O 2 Photosynthesis.The successful synthesis of photocatalysts with abundant active sites, increased specific surface areas, intensified charge carrier generation and transfer, enhanced light absorption, and appropriate band gaps enables the exploration of their H 2 O 2 photosynthetic activities under visible-light irradiation.H 2 O 2 photosynthesis was evaluated with a home-built photocatalytic reactor (Figure S9), with sufficient gaseous O 2 supply in aqueous solutions.The produced H 2 O 2 amount was determined using the wellestablished cerium sulfate Ce(SO 4 ) 2 chromogenic method, for which the calibration relationship between the peak absorbance and the H 2 O 2 concentration was predetermined (Figure S10).The pristine C 3 N 4 -hb sample displays a negligible H 2 O 2 synthesis activity, while the thermally treated carbon nitride samples present significantly increased H 2 O 2 production rates.The H 2 O 2 production rate is dependent on the treatment temperature and time (Figure S11).The performance of the three representative photocatalysts is shown in Figure 6a,b.Among all the samples, C 3 N 4 -nv gives the optimal H 2 O 2 photosynthetic rate of 75.66 μmol g −1 h −1 , which is nearly 45 times higher than that of the pristine C 3 N 4hb sample.In contrast, C 3 N 4 -hb-nv exhibits an H 2 O 2 production rate of 15.90 μmol g −1 h −1 , nearly 9.5 times higher than that of the pristine C 3 N 4 -hb sample.The enhanced photocatalytic activities of C 3 N 4 -hb-nv and C 3 N 4 -nv are associated with their improved light-harvesting capacities, increased specific surface areas, and abundant reactive sites.
Control experiments confirm that H 2 O 2 cannot be generated in the absence of any one of the following conditions: the catalyst, light irradiation, or O 2 (Figure S12).
The dependence of the H 2 O 2 photosynthesis activities on the light absorption of the photocatalysts was further studied by acquiring the action spectrum (Figure 6c and Table S4).The H 2 O 2 photosynthesis was performed with C 3 N 4 -nv under monochromatic light irradiation at different wavelengths.The apparent quantum efficiency (AQE) of the H 2 O 2 photosynthesis at each wavelength was determined from the ratio of the electron number involved in the H 2 O 2 photosynthesis to the incident photon number.The electron number involved in the photocatalytic reaction is twice the number of the produced H 2 O 2 molecules.The variation trend of the AQEs is in good agreement with the absorption spectrum of C 3 N 4 -nv.The AQE reaches as high as 1.90% at 420 nm.The LCCE is also used to evaluate the photon utilization and light energy conversion efficiency.A high LCCE value of 3.85% was obtained with C 3 N 4 -nv for the H 2 O 2 photosynthesis in pure water under visible-light irradiation.It is the highest in comparison with those reported recently (Table S5).The stability of the C 3 N 4nv photocatalyst was evaluated by performing the H 2 O 2 photosynthesis in five cycles.The H 2 O 2 synthesis rate after five cycles with each photocatalytic reaction time of 120 min retained ∼55% of that of the original photocatalyst (Figure 6d).The C 3 N 4 -nv photocatalyst after reaction was therefore characterized (Figure S13).The FTIR spectra show that the peak at 2180 cm −1 , resulting from the formation of −C�N groups, became weaker after reaction.This means that the −C�N groups might be destructed after reaction.The EPR spectra also reveal a decrease in the concentration of NVs after reaction.The sample turned from orange to light yellow, suggesting that their light absorption decreased after reaction.All the results for the catalyst characterization after reaction show that the catalyst deactivated after reaction.The active sites might be attacked by hot electrons, hot holes, and reactive oxygen species, such as hydroxyl radicals. 42Moreover, the mass loss caused by centrifugation after each cycle can also lead to the decrease in the photocatalytic activity.
The photostationary concentration of H 2 O 2 is known to be determined by the competition between the formation rate (k f ) and decomposition rate (k d ) of H 2 O 2 over the catalyst.The overall H 2 O 2 photosynthesis can be calculated by where the reaction kinetics was acquired by assuming the corresponding zeroth order for k f and first-order reaction for k d (Figure 6e,f and Table S6).In terms of the fitting results, C 3 N 4 -nv exhibits a larger k f and smaller k d , which is consistent with its higher overall H 2 O 2 photosynthesis rate than the other two counterparts.
Understanding the Mechanism of the H 2 O 2 Photosynthesis.In the conducted photocatalytic experiments, H 2 O 2 was detected as a product only upon the addition of O 2 gas, indicating that it can be the reduction product from O 2 by hot electrons.The oxidation product by hot holes, on the other hand, is predicted to be O 2 gas.To confirm the formation mechanisms of these two redox products in the half-redox reactions, separate experiments were carried out.The oxidation product was confirmed with C 3 N 4 -nv in an aqueous solution in the presence of AgNO 3 as an electron acceptor and saturated Ar gas bubbling under visible-light irradiation.The gaseous product was detected by gas chromatography with increasing reaction time (Figure S14).The produced O 2 gas increased in amount with the reaction time, confirming that the oxidation product is O 2 from H 2 O molecules (Figure 7a).During this process, C 3 N 4 -nv shows a negligible H 2 O 2 production rate (Figure 7b), indicating that H 2 O 2 is produced through the reduction of O 2 by the photoexcited electrons instead of the oxidation of H 2 O molecules.O 2 gas was therefore produced from the water oxidation by hot holes.Introducing a hole scavenger agent, such as methanol, negligibly affects the H 2 O 2 production rate on C 3 N 4 -nv, further confirming that H 2 O 2 is not produced from the water oxidation but from the oxygen reduction.After the introduction of methanol, a little increase in the H 2 O 2 production rate can sometimes be observed and this might originate from the increased charge carrier transfer.Moreover, possible intermediate radicals involved in the H 2 O 2 synthesis were probed to determine the reaction path (Figure 7b).Superoxide radicals are considered as potential intermediate species in H 2 O 2 synthesis, which can be captured by p-benzoquinone.However, the introduction of pbenzoquinone leads to a little increase in the H 2 O 2 production rate, indicating that the reduction of O 2 to H 2 O 2 undergoes a direct one-step two-electron reduction process without the assistance of superoxide radicals.The increase is highly believed to come from the inhibition of C 3 N 4 -nv degradation by p-benzoquinone.p-Benzoquinone might protect the active sites of C 3 N 4 -nv from being attacked by reactive oxygen species.Furthermore, hydroxyl radicals also play a crucial role in the H 2 O 2 production rate.The decomposition of H 2 O 2 leads to the generation of hydroxyl radicals, as shown in the following equation: If the hydroxyl radicals on the right side of the equation are consumed, the forward reaction will be promoted, leading to a decrease in the gross H 2 O 2 concentration as well as the net H 2 O 2 production rate.The consumption of hydroxyl radicals can effectively be achieved by t-butanol, a chemical known to trap these radicals.As a result, the introduction of t-butanol reduces the production rate of H 2 O 2 (Figure 7b).
To further confirm whether hydroxyl radicals and superoxide radicals exist or not, EPR spectroscopy was also performed (Figure 7c−e and Figure S15).The results indicate the absence of superoxide radicals on the three representative catalysts, implying that they are not intermediate species in the oxygen reduction to H 2 O 2 .These EPR results align with the experimental results of the superoxide radical trapping with the scavenger agents in Figure 7b.C 3 N 4 -hb, C 3 N 4 -hb-nv, and C 3 N 4 -nv all pass through the one-step two-electron reduction pathway without the generation of the intermediate superoxide radicals.As described in the Introduction section, the formation of 1,4-endoperoxide species on the C 3 N 4 -hb surface suppresses the undesired one-electron ORR to •OOH but preferentially accelerates the selective two-electron ORR to H 2 O 2 .The selectivity of the two-electron ORR toward H 2 O 2 production over C 3 N 4 -hb is therefore high. 28The chemical structure of C 3 N 4 -hb-nv shows that it also possesses the tri-striazine units connected through planar amino groups (Figure 3g).Therefore, 1,4-endoperoxide species should also form on C 3 N 4 -hb-nv, which can suppress the undesired one-electron ORR to •OOH but preferentially accelerate the selective twoelectron ORR to H 2 O 2 .C 3 N 4 -hb-nv should also show a high selectivity of the two-electron ORR toward H 2 O 2 production, although C 3 N 4 -hb-nv has the suitable band position for the generation of superoxide radicals.In contrast, hydroxyl signals are significantly decreased to a nearly negligible level in C 3 N 4nv compared with the other two samples (Figure 7c−e).This observation suggests that the H 2 O 2 decomposition is greatly inhibited for C 3 N 4 -nv, which agrees well with the lower k d values shown in Figure 6f.
Overall, O 2 gas is reduced into H 2 O 2 through a one-step two-electron reduction reaction.The average electron transfer number (n) involved in the ORR process in the three samples was studied in the O 2 -saturated electrolyte by use of a rotating disk electrode (RDE) at different rotation speeds (Figure 7f− h).The diffusion-limited current density rises with the increase in the rotation speed due to the enhanced oxygen reduction kinetics with faster O 2 diffusion.The electron transfer number can be calculated from the Koutecky−Levich plots derived from the RDE measurements at different rotation speeds (Figure S16).The electron transfer numbers were determined to be 2.272, 1.804, and 2.001 at 0.25 V vs RHE for C 3 N 4 -hb, C 3 N 4 -hb-nv, and C 3 N 4 -nv, respectively.The H 2 O 2 formation catalyzed by C 3 N 4 -hb, C 3 N 4 -hb-nv, and C 3 N 4 -nv is therefore dominated by the one-step two-electron ORR pathway.
To obtain deeper insights into the reaction mechanism, DFT calculations were further performed.The adsorption and activation of O 2 on the catalyst surface is a key step in twoelectron ORR.The adsorption configurations of an O 2 molecule on the C 3 N 4 -hb, C 3 N 4 -hb-nv, and C 3 N 4 -nv models are shown in Figure 8a.The O 2 molecule was found to adsorb on the tri-s-triazine unit of C 3 N 4 -hb, on the NV of C 3 N 4 -hb-nv, and on the NV and −C�N group of C 3 N 4 -nv.The adsorption of O 2 on C 3 N 4 -hb is determined to be an endothermic process, indicating that the adsorption of O 2 on C 3 N 4 -hb is unstable.In contrast, C 3 N 4 -hb-nv and C 3 N 4 -nv produce stable adsorption of O 2 , with adsorption energies of −0.33 and −0.57eV, respectively.Hence, the existence of NVs and −C�N groups promotes the adsorption of O 2 , which facilitates the subsequent reduction of O 2 .The charge density difference (CDD) and localized density of states (LDOS) were employed to study the electronic structure changes after O 2 adsorption.The CDD analysis shows that the adsorption of O 2 on C 3 N 4hb-nv and C 3 N 4 -nv results in more prominent charge transfer than on C 3 N 4 -hb (Figure 8b), which is consistent with the variation trend of the adsorption energy.The LDOS analysis demonstrates a substantial splitting of the σ 2p , π 2p , and π 2p * molecular orbitals of O 2 upon adsorption on C 3 N 4 -hb-nv and C 3 N 4 -nv, while the variation is negligible for O 2 adsorbed on C 3 N 4 -hb (Figure 8c).This splitting gives rise to purely empty states for the adsorbed O 2 , favoring the acceptance of photoexcited electrons for reduction.
The energy diagrams for the reduction of O 2 into H 2 O 2 on C 3 N 4 -hb, C 3 N 4 -hb-nv, and C 3 N 4 -nv are shown in Figure 8d.Owing to the strong adsorption and activation of O 2 , the reduction of O 2 into H 2 O 2 is a pure exothermic process on C 3 N 4 -hb-nv and C 3 N 4 -nv.In contrast, C 3 N 4 -hb accounts for an endothermic adsorption step.Hence, C 3 N 4 -hb-nv and C 3 N 4 -nv give rise to higher H 2 O 2 production rates than C 3 N 4 -hb.
The subsequent hydrogenation of the adsorbed O 2 molecules, which is also crucial for H 2 O 2 formation, was further studied through 1 H nuclear magnetic resonance ( 1 H NMR) spectra (Figure 8e).The 1 H NMR spectra exhibit two prominent peaks at ∼9.1 ppm (H b ) and 4.0 ppm (H a ) for the three samples, which are ascribed to amino groups and residual water, respectively.The relative intensity of H a :H b increases clearly from C 3 N 4 -hb to C 3 N 4 -hb-nv and further to C 3 N 4 -nv.This indicates the increase of amino groups from C 3 N 4 -hb to C 3 N 4 -hb-nv and further to C 3 N 4 -nv, resulting in the increase of hydrophilicity from C 3 N 4 -hb to C 3 N 4 -hb-nv and further to C 3 N 4 -nv because of the formation of hydrogen bonds between amino groups and H 2 O in the solution.The H b peak is shifted to the lower field, which can be ascribed to the formation of the electron-withdrawing groups of −C�N.Contact angle measurements also show that C 3 N 4 -nv is superhydrophilic.When a water droplet was released on the surface of C 3 N 4 -nv, it quickly infiltrated the surface and spread out completely, reaching a contact angle of 0°within 0.24 s.In contrast, when a droplet was released on the surface of C 3 N 4 -hb-nv and C 3 N 4hb, the contact angle decreased to 36.2 and 48.0°within 0.27 s, respectively.The water droplet spread out completely within a longer time for C 3 N 4 -hb-nv and C 3 N 4 -hb (Figures S17 and  S18).The contact angle measurements confirm the increase of hydrophilicity from C 3 N 4 -hb to C 3 N 4 -hb-nv and further to C 3 N 4 -nv.Moreover, the zeta potentials of the C 3 N 4 -hb, C 3 N 4hb-nv, and C 3 N 4 -nv samples dispersed in water were further measured (Figure S19).The results show that the hydrogen bond breaking on carbon nitride leads to the formation of more amino groups, which will absorb negatively charged ions in water to form a Stern layer and lead to more negative zeta potentials.Namely, the interaction between carbon nitride and H 2 O molecules becomes stronger with hydrogen bonding breaking on carbon nitride.
According to the combined experimental and theoretical calculation results above, a creditable mechanism centered around C 3 N 4 -nv is put forward (Figure 8f).C 3 N 4 -nv first absorbs photons to generate electron−hole pairs under visiblelight irradiation.The photogenerated electrons transfer and accumulate on the defect sites such as the NVs and −C�N groups.These defect sites are the active sites for the efficient adsorption and activation of O 2 toward H 2 O 2 .The interaction between the hydrogen-bond-broken carbon nitride surface and O 2 becomes stronger.The adsorbed and activated O 2 molecule undergoes a one-step two-electron reduction reaction by the photogenerated electrons to produce H 2 O 2 , while H 2 O molecules are oxidized to O 2 gas by the photogenerated holes.The H 2 O 2 decomposition is also inhibited on C 3 N 4 -nv to some extent.Moreover, the hydrogen-bond-broken carbon nitride was found to have more exposed N atoms, which can form hydrogen bonds with H 2 O molecules in the solution.The exposed N atoms serve as proton buffering sites, which facilitates the proton donation.In short, the hydrogen bonds between the exposed N atoms and H 2 O molecules contribute to the enhanced proton-coupled electron transfer.Strong hydrogen bonds increase the proton tunneling dynamics by a factor of 10−10 3 . 36Hydrogen migration therefore proceeds from the exposed N atoms to the NVs or −C�N groups.

CONCLUSIONS
In summary, the interactions between the catalyst surface and reactants, including O 2 and H 2 O molecules, during H 2 O 2 photosynthesis are facilitated through the intralayer hydrogen bond breaking in carbon nitride.The intralayer hydrogen bond breaking in carbon nitride is achieved by post-thermal treatment in an inert atmosphere.The as-obtained hydrogenbond-broken carbon nitride exhibits enhanced light absorption, increased specific surface areas, abundant active sites, and increased charge carrier densities.The hydrogen-bond-broken carbon nitride delivers greatly enhanced photocatalytic H 2 O 2 production rates and increased AQE and LCCE values.The reaction mechanism study shows that the strengthened interactions between the catalyst surface and reactants play a crucial role.First, the formation of active sites on hydrogenbond-broken carbon nitride including NVs and cyano groups contributes to the increased adsorption and reduction of O 2 molecules.O 2 molecules are reduced by photogenerated electrons in a one-step two-electron reduction pathway.In addition, the interaction between the catalyst surface and H 2 O molecules is also promoted owing to the hydrogen bond formation between the exposed N atoms on hydrogen-bondbroken carbon nitride and H 2 O molecules.The exposed N atoms serve as proton buffering sites, which contribute to the enhancement of proton-coupled electron transfer.Moreover, H 2 O is oxidized to give O 2 by the photogenerated holes.The decomposition of H 2 O 2 is also suppressed.Our study provides an attractive method to enhance the interaction between the catalyst surface and reactants and proton donation through the hydrogen bond formation between catalysts and H 2 O molecules.

METHODS
Catalyst Preparation.The synthesis of the hydrogen-bondbroken carbon nitride samples involved three steps.The first step was the synthesis of bulk g-C 3 N 4 , which was carried out through the thermal condensation of melamine in air in a muffle furnace.Melamine powder was first placed in a lidded alumina crucible.The furnace was heated to 550 °C at a heating rate of 10 °C min −1 and then kept at 550 °C for 6 h.Bulk g-C 3 N 4 was obtained after the muffle furnace cooled down to room temperature. 27The second step was the thermal exfoliation of the as-prepared bulk g-C 3 N 4 into g-C 3 N 4 nanosheets.The thermal exfoliation process proceeded in air in a muffle furnace, with the bulk g-C 3 N 4 sample placed in an unlidded alumina crucible and kept at 500 °C for 6 h for complete exfoliation. 43he g-C 3 N 4 nanosheets were obtained after thermal exfoliation.The third step was the thermal treatment of the thermally exfoliated g-C 3 N 4 nanosheets in Ar gas in a tube furnace.The flow rate of Ar gas was set at 50 mL min −1 .The treatment temperature ranged from 520 to 650 °C, with the treatment time varied from 2 to 10 h.When the treatment temperature was increased above 650 °C and the treatment time was prolonged, the hydrogen bonds in the thermally exfoliated g-C 3 N 4 nanosheets were completely destroyed. 34 2 O 2 Photosynthesis.The photocatalytic H 2 O 2 production was performed at room temperature and pressure.A homemade reactor was used.Two openings on the reactor were used for the input and output of O 2 , respectively.For the photocatalytic H 2 O 2 production, the photocatalyst (50 mg) was dispersed into deionized water (50 mL) in the photocatalytic reactor through mild ultrasonication for 15 min.O 2 gas was subsequently bubbled into the photocatalyst suspension at a constant flow rate of 50 mL min −1 under magnetic stirring.The bubbling process lasted for ∼30 min to ensure the complete adsorption of O 2 on the photocatalyst and dissolution of O 2 into the suspension.A 300 W xenon lamp was then turned on to irradiate the reaction solution.Different filters were inserted in the light filter slot to alter the spectral range of the irradiating light.A 420 nm cutoff filter was employed to obtain the visible light, while an AM 1.5G solar light filter was used to simulate sunlight.Various band-pass filters were utilized to create monochromatic light.The light intensity, which was measured with a pyranometer (KIPP&ZONEN CMP3), was controlled by adjusting the intensity button on the xenon light source and the distance between the light source and the reaction solution.The production of H 2 O 2 with reaction time was monitored by collecting a portion of the reaction solution (3.0 mL) every 20 min, including one portion before the reaction and many portions during the reaction.The collected reaction solution was then immediately centrifuged at 8000 rpm for 2 min to remove the photocatalyst, followed by the filtration with a sterile syringe filter with a 0.22 μm disposable membrane placed inside.
Evaluation of the Photocatalytic H 2 O 2 Production.The produced H 2 O 2 was measured by a traditional cerium sulfate Ce(SO 4 ) 2 colorimetric method. 44Ce(SO 4 ) 2 was utilized as an indicator for H 2 O 2 owing to the reduction of yellow-colored Ce 4+ ions to colorless Ce 3+ ions according to the following equation:

ACS Nano
The concentration of Ce 4+ can therefore be measured by ultraviolet/ visible absorption spectroscopy.The wavelength at 316 nm was the characteristic absorption peak of Ce 4+ ions in the absorption measurements.The concentration (c) of H 2 O 2 was determined according to the following equation: Specifically, a Ce(SO 4 ) 2 aqueous solution (1 mM) of a yellow transparent color was prepared by dissolving 33.2 mg of Ce(SO 4 ) 2 into a sulfuric acid solution (0.5 M, 100 mL).The calibration curve was obtained by measuring the absorption spectra of Ce(SO 4 ) 2 solutions at different concentrations, which were preprepared by diluting an as-prepared Ce(SO 4 ) 2 solution.According to the linear relationship between the absorption peak intensity and the Ce 4+ concentration, the H 2 O 2 concentration of the measured sample solution can be determined.The production rate of H 2 O 2 is an intuitive parameter for evaluating the photocatalytic H 2 O 2 production performance.However, the production rate varied with different conditions, such as the light intensity and the O 2 gas feeding rate.The AQE and LCCE were therefore employed to evaluate the effects caused by these parameters and quantify the photocatalytic H 2 O 2 production performance.The AQE and LCCE were used to describe the photocatalytic H 2 O 2 production performance from microscopic and macroscopic perspectives, respectively.AQE evaluates the incident photon utilization efficiency and is defined as the following equation: The reacted electron number (N reacted electrons ) indicates the molar number of the photosynthesized H 2 O 2 , that is, that one mole of H 2 O 2 represents two moles of transferred electrons in the reaction.Moreover, to obtain the incident light photon number (N incident photons ), monochromic light sources were employed.The incident light photon number was calculated according to the following equation: where ΔG is 117 kJ mol −1 for the photocatalytic H 2 O 2 production from water and oxygen gas (O 2 + 2H 2 O → 2H 2 O 2 ).The requirement for the LCCE estimation is that the photocatalytic reaction is free from sacrificial agents.In the LCCE evaluation process, the catalyst (50 mg) and deionized water (50 mL) were added into a home-built reactor, with a cutoff filter (λ > 420 nm) inserted to simulate the visible light.The light intensity was adjusted to 1000 W m −2 .The cutoff filter (λ > 420 nm) was used to inhibit H 2 O 2 decomposition.Characterization.TEM characterization was performed on FEI Tecnai Spirit operated at 120 kV.XPS characterization was carried out on Thermo Fisher ESCALAB 250Xi.XRD measurements were performed on an X-ray diffractometer (RU-300, Rigaku) with Cu Kα radiation (λ = 1.5406Å) at room temperature in air.FTIR spectra were recorded on a Thermo Nicolet NEXUS 670 spectrometer.EELS measurements were performed on an FEI Tecnai F20 microscope.Solid-state 1 H and 13 C NMR spectra were acquired on Bruker WB 400 M to characterize the molecular structures of the samples by measuring the interaction of nuclear spins under a magnetic field.N 2 adsorption−desorption isotherms were measured on a Micromeritics ASAP 2020 analyzer.The specific surface areas of the samples were calculated according to the Brunauer−Emmett−Teller (BET) method.Low-temperature EPR spectra were acquired on a Bruker EMX EPR spectrometer (BioSpin GmbH).TGA was conducted on a PerkinElmer system with a heating rate at 10 °C min −1 .O 2 -TPD was performed on TP-5080 in He gas, with the heating rate set at 10 °C min −1 .The absorption spectra for the determination of the generated H 2 O 2 and the absorption spectra of the powder samples were measured on an ultraviolet/visible/near-infrared spectrophotometer (PerkinElmer Lambda 950).Steady-state PL spectra were recorded on a Hitachi F-4600 spectrophotometer.Transient PL spectra were taken on a HORIBA FluoroMax-4 fluorometer.Electrochemical measurements were carried out on an electrochemical workstation (Shanghai Chenhua CHI760E).Contact angle measurements were performed on DataPhysics OCA20.Zeta potentials were measured with DLS-Malvern Instrument Zetasizer ZS90.
Density Functional Theory Calculations.The DFT calculations were performed using the commercial Vienna Ab initio Simulation Package (VASP). 45,46In the calculations, the ion-electron interaction was described by the projector-augmented wave (PAW) method. 47The density functional was treated by the generalized gradient density approximation (GGA) with the Perdew−Burke− Ernzerhof (PBE) exchange-correlation potential. 48The adsorption configurations of O 2 on C 3 N 4 -hb, C 3 N 4 -hb-nv, and C 3 N 4 -nv were obtained through complete geometry optimization.The adsorption energy was calculated according to where E ads is the adsorption energy, E O2/catal is the energy of the photocatalyst adsorbed with O 2 , E O2 is the energy of free O 2 , and E catal is the energy of the bare photocatalyst.

Figure 1 .
Figure 1.Atomic structure of layered g-C 3 N 4 with intralayer hydrogen bonds.(a) Side view.(b) Top view.The C, N, and H atoms are represented by the dark-gray, dark-blue, and white balls, respectively.The melon strands are marked by the light pink triangles.(c) XRD pattern of g-C 3 N 4 .

Figure 3 .
Figure 3.Chemical structures of C 3 N 4 -hb, C 3 N 4 -hb-nv, and C 3 N 4 -nv.(a, b) High-resolution C 1s (a) and N 1s (b) XPS spectra.(c, d) Percentages and ratios of the three peaks in the C 1s (c) and N 1s (d) XPS spectra.(e) Atomic ratios of N:C determined from elemental analysis.(f) EELS results revealing the C K and N K edges with the σ* and π* resonances.(g) Derived chemical structures of C 3 N 4 -hb, C 3 N 4 -hb-nv, and C 3 N 4 -nv.
−1 is ascribed to the overlapping vibrations of −OH stretching (C−OH groups and adsorbed H 2 O) and the stretching vibrations of the −NH and −NH 2 groups.The primary change observed after thermal treatment is the emergence of a new peak at 2180 cm −1 , resulting from the formation of −C�N groups through the deprotonation of −C−NH 2 .
Figure 3c,d shows the relative distributions of the different groups.The percentage of C�N−C in the N 1s peak gradually decreases from C 3 N 4 -hb to C 3 N 4 -hb-nv to further C 3 N 4 -nv, suggesting the progressive destruction of the C�N−C groups upon the thermal treatment.The percentage of the NH x groups in the N 1s peak initially decreases in C 3 N 4 -hb-nv and then slightly increases in C 3 N 4 -nv.Nevertheless, the amounts of the NH x groups in both C 3 N 4 -hb-nv and C 3 N 4 -nv are lower than that in C 3 N 4 -hb, which indicates a reduction in the quantity of the NH x groups as well as their breakdown.The atomic ratio of N:C, as determined by elemental analysis, decreases from 3.36:3 for C 3 N 4 -hb to 3.05:3 for C 3 N 4 -hb-nv and further to 2.80:3 for C 3 N 4 -nv (Figure3e).It should be noted that the residual NH x groups are responsible for the formation of intralayer hydrogen bonds.As the NH x groups are more susceptible to destruction compared to N 2C and N 3C , the decreased N:C ratios are attributed to the loss of the NH x and C�N−C groups.Therefore, the XRD and XPS results collectively indicate the destruction of the NH x and C�N− C groups, accompanied by the breaking of the intralayer hydrogen bonds during thermal treatment.The 13 C magicangle spinning nuclear magnetic resonance (MAS NMR) spectra further confirm their chemical structures (FigureS5).Two major peaks located at about 164 and 156 ppm can be observed from the13 C MAS NMR spectra.They can be ascribed to the chemical shifts of C 2N-NHx (C3) and C 3N (C2),

Figure 4 .
Figure 4. Band structures of C 3 N 4 -hb, C 3 N 4 -hb-nv, and C 3 N 4 -nv.(a) Light absorption spectra.(b) Tauc plots derived from the corresponding light absorption spectra.(c) XPS VB spectra.(d) Determined band-edge positions.(e) Schematic illustrating the electronic band structure of C 3 N 4 -nv in comparison with the redox potentials of the involved reactions.
C 3 N 4 -hb and C 3 N 4 -hb-nv, hydrogen bonds are present, while in C 3 N 4 -nv, hydrogen bonds are completely broken.In C 3 N 4 -hb-nv, NVs are formed at the locations of the C�N−C and NH x groups, which are not connected by hydrogen bonds.Similarly, in C 3 N 4 -nv, NVs are also formed at the locations of the C�N−C and NH x groups, but in this case, C�N−C and NH x groups are connected by hydrogen bonds as observed in C 3 N 4 -hb-nv.In addition, −C�N groups are formed in C 3 N 4 -nv due to the deprotonation of −C−NH 2 .
FigureS6.In comparison with C 3 N 4 -hb, the thermally treated samples exhibit substantially reinforced light absorption, with a pronounced redshift of the absorption edge into the nearinfrared region as the treatment temperature and duration are increased.Moreover, in C 3 N 4 -hb, the light absorption is attributed to the intrinsic electronic transition from π to π*.The enhanced π−π* electronic transition can be ascribed to the expanded π-conjugated aromatic framework and better packing of the joint heptazine system.Notably, in C 3 N 4 -nv, a new absorption peak around 490 nm emerges, which is attributed to the n to π* electronic transition of the lone pair electrons at the defect sites (−C�N) (Figure4a).41Their electronic band gap was determined using the Tauc relationship derived from the absorption spectra (Figure4b).The calculated band gaps are 2.38, 2.38, and 1.65 eV for C 3 N 4 -hb, C 3 N 4 -hb-nv, and C 3 N 4 -nv, respectively.The XPS valence band (VB) spectra in Figure4cdisplay the well-determined VB edge positions for C 3 N 4 -hb, C 3 N 4 -hb-nv, and C 3 N 4 -nv at 2.33, 2.11, and 1.85 eV, respectively.The VB potential values show a gradually decreasing tendency.The VB edge positions were further converted to potentials relative to the normalized hydrogen electrode (NHE) according to = + VB(against NHE) VB energy 4.44

Figure 5 .
Figure 5. Properties of C 3 N 4 -hb, C 3 N 4 -hb-nv, and C 3 N 4 -nv.(a) O 2 TPD profiles.(b) EPR spectra.(c) EIS Nyquist plots.(d) Photocurrent responses in Ar and O 2 atmospheres with the visible-light irradiation switched on and off repeatedly.(e) Steady PL spectra with excitation at 330 nm.(f) Transient PL decay spectra with excitation at their absorption edge.The red, blue, and green solid lines are the fitting results of the decay spectra.
), indicating the existence of NVs.Notably, C 3 N 4 -nv exhibits the strongest EPR signal while C 3 N 4 -hb displays the weakest signal.The NVs are capable of electron redistribution to the adjacent C atoms within the delocalized π-conjugated network.The increase in the EPR signal with the increase in the treatment temperature indicates an elevated concentration of unpaired electrons within the aromatic rings.This higher concentration of delocalized unpaired electrons facilitates electron transfer to reactive adsorbates in the catalytic reactions.The abundant NVs offer plentiful chemisorption and activation sites for O 2 molecules, promoting O 2 reduction by electrons trapped at the NV sites.The higher concentration of delocalized electrons leads to reduced charge transfer resistances, as revealed by the smaller impedance arc radii observed in the electrochemical impedance spectra (EIS, Figure5c).The photocurrent responses of the three photocatalyst samples were measured under visible light in both Ar and O 2 atmospheres (Figure5d).The absolute photocurrents increase in the order of C 3 N 4 -hb, C 3 N 4 -hb-nv, and C 3 N 4 -nv in both Ar and O 2 atmospheres, which is consistent with the impedance differences.Compared with those under an Ar atmosphere, the photocurrents of C 3 N 4 -hb, C 3 N 4 -hb-nv, and C 3 N 4 -nv are reduced by 14.1, 15.5, and 54.9% in the O 2 atmosphere, respectively (Figure5d, right side).The increased photocurrent differences between Ar and O 2 are ascribed to the enhanced electron consumption by O 2 molecules at the active sites.The presence of NVs is beneficial for interfacial electron transfer from the photocatalyst to the activated O 2 molecules.To investigate charge carrier recombination, photoluminescence (PL) measurements were performed (Figure5e,f).Compared with C 3 N 4 -hb, C 3 N 4 -hb-nv shows a clear decrease in PL intensity and C 3 N 4 -nv exhibits negligible PL emissions (Figure5e).This result suggests that radiative recombination of charge carriers is greatly inhibited in C 3 N 4 -hb-nv and C 3 N 4 -nv owing to the presence of the NVs and −C�N groups.Moreover, transient PL spectra were obtained by exciting at their absorption edge and monitoring at their emission peaks.As shown in Figure5fand TableS3, the PL lifetime of C 3 N 4 -nv (64.79 ns) is nearly 10 times longer than that of C 3 N 4 -hb (6.38 ns) and C 3 N 4 -hb-nv (13.73 ns).Overall, C 3 N 4 -nv with complete hydrogen bond breaking and

Figure 6 .
Figure 6.Photocatalytic H 2 O 2 production of C 3 N 4 -hb, C 3 N 4 -hb-nv, and C 3 N 4 -nv.(a) Time courses of the generated H 2 O 2 concentrations under visible-light irradiation (λ > 420 nm).(b) H 2 O 2 production rates.The error bars in (a, b) represent one standard deviation.(c) Light absorption (left axis) and AQE action (right axis) spectra of C 3 N 4 -nv.(d) Cycling tests for H 2 O 2 production with C 3 N 4 -nv.(e) Photocatalytic decomposition of H 2 O 2 with C 3 N 4 -hb, C 3 N 4 -hb-nv, and C 3 N 4 -nv under visible-light irradiation.(f) H 2 O 2 formation rate constant (k f ) and decomposition rate constant (k d ) over C 3 N 4 -hb, C 3 N 4 -hb-nv, and C 3 N 4 -nv.

Figure 7 .
Figure 7. Mechanism investigation.(a) Oxidation product detection by gas chromatography.(b) Photocatalytic H 2 O 2 production rates in the presence of different scavenger agents under visible-light irradiation.(c−e) EPR spectra of •OH generated with C 3 N 4 -hb, C 3 N 4 -hb-nv, and C 3 N 4 -nv under visible-light irradiation, respectively.(f−h) Linear sweep voltammetry curves of C 3 N 4 -hb, C 3 N 4 -hb-nv, and C 3 N 4 -nv measured on RDE at different rotating speeds, respectively.

Figure 8 .
Figure 8. Mechanism investigation by DFT calculations.(a) Adsorption configurations of an O 2 molecule on the surface.The adsorption energy is shown at the bottom of the adsorption configuration.(b) CDD of adsorbed O 2 .(c) LDOS for the O 2 molecule after adsorption on the surface of C 3 N 4 -hb, C 3 N 4 -hb-nv, and C 3 N 4 -nv.(d) Energy diagram for the H 2 O 2 synthesis on the different photocatalysts.(e) Solid-state 1 H MAS NMR spectra.(f) Proposed reaction mechanism.The blue, gray, white, red, and pink spheres represent the N, C, H, O atoms, and NVs, respectively.
LCCE was also employed to quantify the photocatalytic H 2 O 2 production performance, which is defined according to the following equation: O 2 (eq 4).This is the competitive reaction to the H 2 O 2 generation from the reduction of the intermediate •O 2 − .
C MAS NMR spectra, light absorption spectra, Mott-Schottky curves, and thermogravimetric analysis of the carbon nitride samples; photograph of the reactor; calibration relationship for the determination of the H 2 O 2 concentrations; photocatalytic H 2 O 2 production under different conditions; C 3 N 4 -nv photocatalyst before and after reaction; schematic showing the determination of the oxidation product; EPR spectra; Koutecky−Levich plots at different potentials; contact angle images and variations; zeta potentials; binding energies of C 1s and N 1s core electrons; PL decay time values; and determined AQEs and k f and k d values; comparison of photocatalytic H 2 O 2 production performances (PDF)